Method of determining formation porosity during drilling



Raw;

METHOD OF DETERMINING FORMATION POROSITY DURING DRILLING D R A w Y A H J. 5 4 9 1 M H 2 :m M r m M 5 1 Filed April 20, 1942 INVENTOR JOHN T. HAYWARD ATTORNEY ssmuunttsmi Patented Mar. 27, 1945 UNITED STATES PATENT OFFICE METHOD OF DETERMINING FORMATION POROSITY DURING DRILLmG 2 Claims.

This invention relates to a method for determining porosities of earth formations during drilling of wells by the rotary method, and particularly to a method for determining the porosity of oilor gas-containing formations during rotary drilling.

The porosity of earth formations encountered in the drilling of oil and gas wells, and particularly the porosities of oiland gas-containing formations, has become a subject of increasingly great importance to the oil industry, since it is a prime factor in the estimation of petroleum reserves in place in oil and gas fields and in the determination of the probable productivity of such fields.

Heretofore the only method of determining the porosity of such a formation, which has been found acceptable, is that of core analysis, wherein a solid core sample of the formation, removed by one of the conventional core drilling methods,

is examined by physical methods to determine its porosity. This method has many disadvantages, due primari1y to the mechanical difliculty of cutting and removing from the well a continuous and unbroken section of the formation whose porosity is sought. Porous formations, particularly those containing oil or gas, are frequently loose, unconsolidated, sandy or shaly structures which undergo considerable crumbling and disintegration during the core drilling operation and under the washing action of the drilling fluid usually circulated during the drilling. As a result, core recoveries are seldom complete and from the core recovered only small selected pieces are normally tested for porosity. The final porosity determinations are, therefore, usually based on figures obtained from perhaps one percent or less of the total vertical section drilled, and such determinations are necessarily of questionable accuracy as applied to the entire section drilled. In addition, the coring operation requires special tools and necessitates frequent interruptions in the normal drilling operations which are hazardous to the successful completion of the well, and in many cases, have caused a well to be lost.

This invention is based upon the discovery that because of the usually negligible porosity of the usual hard cap rock which normally overlies the relatively porous formation constituting the usual pay formation, the ratio of the drilling rate in the cap rock to that in the adjacent pay formation may be employed to determine the porosity of the pay formations. As a result of this discovery, it is found that the porosities of such porous formations may be determined directly from the drilling rates, during the course of conventional rotary drilling operations and without interruption of the normal drilling, and, of course, without recourse to core drilling heretofore employed to determine porosities.

By the method in accordance with this invention, the determination of the porosity of a pay formation may be, and usually is, made in a continuous manner for the entire vertical section of the formation as it is drilled, and the porosity value thus determined is found to be much more accurate for the entire section, than that obtained by the more conventional core analysis methods heretofore employed, which, as pointed out above, are necessarily based upon fragmentary portions of the drilled section.

In accordance with this invention, the ratio of the drilling rate through a pay formation to the drilling rate through its cap rock is determined and this ratio, termed the drilling ratio is then co-related with a suitable porosity index which is constructed from such ratios which have been previously established for like formations in similar lithological sections.

For the purposes of this invention, a lithological section is intended to refer to that section of a well comprising a pay formation and its adjacent cap rock. The porosity index referred to is preferably a curve showing the relationship of drilling ratios to porosity in a pay formation similar to that being examined and existing in a similar lithological section. Such curves can be readily constructed on the basis of previous drilling rate and core analysis information which has been previously ascertained for areas where similar lithological sections exist, and such information is readily available to those engaged in the drilling industry. Such curves may be prepared for the various types of pay formations, such as sands, limestones, and the like, for employment in accordance with this invention.

The accompanying drawing illustrates a family of such curves.

In accordance with a general embodiment of this invention, drilling is conducted in the usual manner by the rotary method to advance the drill through the cap rock formation and the immediately underlying pay formation. Precautions are normally taken to maintain substantially uniform drilling conditions during the advancement of the drill through both formations. Tnese drilling conditions include the rotational rate of the drill, the drilling fluid circulation rate, and the weight-on-bit. The drilling rates through the cap rock and the pay formation are measured by any suitable or conventional means, and the ratio between them is determined, preferably by dividing the rate in the cap rock into the rate in the pay formation, in order that the resulting quotient, or drilling ratio, shall be a whole number, for purposes of convenience. By way of example, assume that the drilling rate in the cap rock of a particular lithological section is 3 feet per hour, and in the immediately adjacent pay formation which is a sand, the drilling rate is 30 feet per hour. The drilling ratio R. will be found to be 10. Referring to the chart shown in the drawing, the values of the drilling ratios comprise the vertical axis and porosity (in percent) the horizontal axis. A curve S is a porosity index characteristic of similar sand pay formation in similar lithological sections. The value R is located as indicated, on the drilling ratio axis at the value 10. From this point a line is drawn horizontally to intersect the curve S and from this intersection, a line drawn vertically to the porosity axis will intersect the latter at the point P which gives the total porosity of the unknown pay formation as approximately 30.6 percent.

A curve L is also shown on the drawing to illustrate a, curve constituting a porosity index characteristic of a limestone pay formation and may be employed in a similar manner for determining the porosities of limestone pay formations.

With the method of this invention, it is readily possible to continuously determine the porosity of the entire vertical section of the pay formation being drilled. Thus as the drill advances through the pay formation, the drilling rate may be continuously measured as the drill advances through the formation. Since the drilling rate through the cap rock will have already been determined the drilling ratio may be continuously determined as the drill proceeds through the pay formation, and the corresponding porosity can be determined from the chart. Obviously, if the pay formation is of relatively uniform hardness, the porosity of the entire pay formation will be substantially uniform. On the other hand, if the pay formation varies in hardness, the porosity of each vertical segment thereof may be determined by determining the drilling ratio for each segment as the drill advances, and co-relating these ratios with an applicable porosity index curve. In this way the true porosity value of the entire vertical section of the pay formation, or any segment thereof, may be obtained with much greater accuracy than has heretofore been possible.

As indicated above, it is important, in accordance with this invention, that the measurements of the drilling rates in the cap rock and pay formations be made under uniform drilling conditions of rotational speed, fluid circulation rate, and weight-on-bit. Accordingly, wherever possible or convenient, these conditions should normally be maintained uniform by the use of the usual drilling controls available for this purpose. In such cases, no measurement of these conditions need be made for purposes of correcting the drilling rates, since these factors will remain constant during the drilling of both formations. Frequently, however, practical considerations will require that some of these conditions, particularly the rotary speed or the weight-on-bit, should be changed from one formation to the other. Under these conditions, the values of the changed conditions should be measured for each formation and correction factors supplied to the actual drilling rate measurements in order to correct these measurements for the changed conditions, so that the finally determined drilling rates shall be as though measured under uniform drilling conditions. The effect of changes, particularly in rotational speed and weight-on-bit, on the drilling rates is now known, and suitable mathematical correction factors have been determined and may be readily applied where necessary.

It will be understood that other suitable mathematical functions of the drilling rates may be employed instead of the ratios of the drilling rates, for determining porosity of the formation, provided,- of course, a characteristic porosity index for co-relation purposes has first been established for similar pay formations in terms of those particular functions.

What I claim and desire to secure by Letters Patent is:

1. The method of determining the porosity of an earth formation during the drilling of a well by the rotary system, comprising progressively determining the rates of drilling at substantially uniform drilling progressively conditions, including rotational rate, drilling fluid circulation rate, and .weight-on-bit while drilling through a lithological section comprising a relatively hard formation and an adjacent relatively porous formation, whereby upon dividing the ascertained drilling rate for the relatively porous formation by the ascertained drilling rate for the relatively hard formation, there is established a drilling ratio which is characteristic of the porosity of said relatively porous formation, and co-relating said ratio with a porosity index for said relatively porous formation constructed from such drilling ratios previously established for a like formation in a similar lithological section.

2. The method of determining the porosity of an earth formation during the drilling of a well by the rotary system, comprising, maintaining substantially uniform drilling conditions, including rotational rate, drilling fluid circulation rate and weight-on-bit, while drilling through a lithological section comprising a relatively hard formation and an adjacent relatively porous formation, measuring the rates of drilling through both said formations, whereby upon dividing the ascertained drilling rate for the relatively porous foramtion by the ascertained drilling rate for the relatively hard formation there is established a drilling ratio which is characteristic of the porosity of said relatively porous formation, and co-relating said ratio with a porosity index for said relatively porous formation constructed from such drilling ratios previously established for a like formation in a similar lithological section.

JOHN T. HAYWARD. 

